Hydrogenation of benzene to cyclohexane



United States Patent HYDROGENA' IION 0F BENZENE T0 CYCLOHEXANE Louis S; Kassel, Oak Park, 11]., assignor to Universal Oil Products Company, Des Plaiues, Ill., a corporation of Delaware Application November 10, 1952, Serial No. 319,747

11 Claims. (CL'260-667) This 'invention'relates. to a method of 'manufacturing cyclohexane and'is more particularly related to a method of manufacturing pure cyclohexane, from a starting material of pure'benzene.

It is d'iflicult to obtain pure hydrocarbons from petroleum because petroleum fractions are characterized as being a continuous mixture of various hydrocarbons and their isomers. A comrnon'method of separating liquids is by distillation, however, during distillation of a pctroleum" mixture the boiling point rises almost continuously firom the initial boiling point of the mixture to the end point. The following table shows the boiling points of some of the hydrocarbons. in petroleum. c C

n-Hexane 68.742 Methylcyclopentane 71.812 2,2-dirnethylpentane 79.200 Benzene 80.100 2,4-dirnethylpentane 80.500 Cyclohexane. 80.738 3',3-dimethylpentane -n 86.065 1,1-dimethylcyclopentane 87.846 Z-methylhexane 90.052

It may be expected trom'examination of this table, and from-the known tendency of cyclic hydrocarbons to form azeotropes, that pure cyclohexane cannot be obtained from: petroleum by distillation. This expectation is confirmed by experiment.

It is an object of this invention to produce pure cyclohexane by a novel method which comprises hydrogenating pure benzene to form substantially pure cyclohexane.

In one embodiment the present invention relates to a process wherein benzene is separated from a hydrocarbon mixture and subsequently is hydrogenated to form cyclohexane.

Recent developments in the art of petroleum refining have made possible the separation of pure benzene from a hydrocarbon fraction. Whereas this. was formerly not possible by distillation techniques, it is now being done on a commercial scale by a combination of distillation and. extraction techniques. For example, a hydrocarbon fraction may be separated which contains benzene and the other hydrocarbons boiling in the benzene range. This cut may be taken so that the only aromatic present is benzene. The benzene-containing hydrocarbon fraction is then subjected to an extraction process which separates the benzene contained in the fraction from the normal or isoparatfins and naphthenes contained in the fraction. The benzene thus separated is easily recovered from the solvent by distillation. Benzene obtained by this method is substantially pure having a benzene content of 99.5% or better. The novel process. of this invention provides a means of hydrogenating substantially all of the benzene to cyclohexane having a purity of 99.5% or better.

The method of this invention provides for catalytic hydrogenation of benzene in a substantially adiabatic "ice reaction zone under conditions so regulated that the reaction will start at the lowest possible temperature, that is the lowest temperature at which hydrogenation will occur, and will proceed until substantially all the benzene has been hydrogenated. Since the hydrogenation reaction is exothermic, an increase in temperature will be experienced, and proper choice of conditions is essential to avoid reaching a temperature at which destructive reactions occur. The reactants are withdrawnfrom the reaction zone, cooled, combined with additional benzene and passed to a second reaction zone wherein a similar cycle is repeated. At all times, in the various reaction zones, the temperatures, pressures, and concentrations of the various reactants must be carefully controlled in order for the desired material to be produced.

In a specific embodiment of the present invention pure cyclohexane is produced by passing a mixture of benzene, cyclohexane, and hydrogen gas at a temperature of from about 325 F. to about 400 F; and a pressure of from about p. s. i. to about 500 p. s. i. into contact with a hydrogenation catalyst in an adiabatic reaction zone. In passing through the hydrogenation catalyst contacting zone the temperature of the reactants increases as benzene is hydrogenated to cyclohexane. After substantially complete hydrogenation, the reactants are withdrawn from the contacting zone at atemperature of from about 500 F. to about 600 F. and subsequently cooled. The cooled reactants are cornmingled with more benzene so that the temperature of the mixture is from about 325 F. to about 400 F. and passed into contact with a second bed of hydrogenating catalyst wherein substantially complete hydrogenation occurs and a temperature of from about 500 F.' to about 600 F. is reached. Reactants removed from this contacting zone are again cooled and commingled with benzene to obtain a temperature of from about 325 F. to about 400 F. In each succeeding reaction zone the benzene is added to the inlet and the commingled stream of reactants and cyclohexane is introduced into the reaction zone at a temperature of from about 325 F. to about 400 F. The cyclohexane and hydrogen are passed through the various reaction zones in series and the benzene is passed through in parallel, that is a portion of the total benzene charge is introduced. into the inlet of each. reactionzone. The efliuent stream from the last reaction zone is withdrawn, cooled to a temperature which usually is'from about 70 F. to about F, and passed into a receiver in which the eliiuent separates into a gaseous phase which is substantially pure hydrogen and a liquid phase which. is substantially pure cyclohexane. The gaseous phase is recycled to the inlet of the first contacting zone and. a separately withdrawn cyclohexane stream is divided; a portion passing as final product and another portion passing to the inlet to the first contacting zone.

In a.- four step reaction, the total benzene should be added in four portions, preferably equal portions, one each of said portions to the inlet of each of the'four contacting zones. The hydrogen passes in series through the four contacting zones" as does the recycled cyclo hexane stream. The temperature rise in each reaction zone is limited to an allowable value by limiting the proportion of benzene in the reactant mixture introduced to that zone. The purpose of the recycled cyclohexane is to increase the allowable ratio of benzene to hydrogen in the reactant mixture, and hence to increase the allowance utilization of hydrogen in a single reaction zone. The purpose of the several reaction zones in series is to reduce the quantity of recycle cyclohexane required and the quantity of excess hydrogen.

Following is an example of one manner of operating the process as hereinbefore described. The present process comprises four contacting zones through which hydrogen and cyclohexane pass in series while the total benzene charge is divided into four equal portions, one portion passing to the inlet of each of the four contacting zones. The operation proceeds thus: one quarter of the benzene charge, a quantity of cyclohexane to be later described, and 9 mols of hydrogen per mol of benzene charge are commingled and passed into the first reaction zone at a temperature of 350 F. and a pressure of 400 p. s. i. The first contacting zone, and all subequent contacting zones, are of the adiabatic type which may be defined as contacting zones wherein no heat is added or removed from the contacting zone itself except that which passes in and out with the process streams. The commingled charge enters the first contacting zone at a temperature of 350 F. and passes therethrough, emerging after substantially complete conversion of the benzene at a temperature not in excess of 550 F. The efiluent from the first contacting zone is cooled, and commingled with a second portion of benzene to produce a mixture at a temperature of 350 F. which is passed into the second contacting zone. Effluent from this zone has a benzene content of substantially zero and a temperature not in excess of 550 F. This effluent is cooled, commingled with a third portion of benzene to produce a mixture at a temperature of 350 F. and passed to the third contacting zone. The efiluent from the third contacting zone contains substantially no benzene and is at a temperature not in excess of 550 F. The third contacting zone etfiuent is cooled, commingled with benzene to produce a mixture at a temperature of 350 F. and passed into the final contacting zone. The eflluent from the fourth contacting zone is cooled to a temperature of about 100 F. and passed into a receiver wherein it is separated into two phases, a hydrogen phase which may be recirculated to the first contacting zone as a portion of the hydrogen necessary for the reaction, and a cyclohexane phase which is divided into two equal portions, one being withdrawn as product and the second passing to the inlet of the first contacting zone as the diluent for the incoming benzene. For the overall process the materials used are in a ratio of one mol of cyclohexane to one mol of benzene to nine mols of hydrogen. The process operated in accordance with this description will produce cyclohexane of a purity in excess of 99% cyclohexane provided pure benzene is used as a starting material. 7

The process of this invention can perhaps best be explained with reference to the accompanying flow diagrammatic drawing which illustrates one embodiment of this invention and is not intended to unduly limit the invention to the particular embodiment illustrated.

Referring now to the drawing, hydrogen and cyclohexane comrningled in line 1 pass to line 3 wherein they are commingled with benzene entering line 3 from line 2. The admixture passes into contacting zone 4 wherein the hydrogenation reactions are effected. The efiluent from contacting zone 4 passes through line 5 and into line 7 wherein the etfiuent from contacting zone 4 is admixed with benzene entering line 7 from line 6. The mixture of benzene and contacting zone 4 efiluent passes into contacting zone 8 wherein further hydrogenation is etfected and the eflluent is withdrawn through line 9 passing to line 11 wherein it is admixed with more benzene entering line 11 from line 10. The mixture in line 11 passes into contacting zone 12 wherein further hydrogenation is effected and is withdrawn from contacting zone 12 through line 13 into line 15 wherein the eflluent from contacting zone 12 is mixed with a final portion of benzene entering line 15 from line 14. The mixture in line 15 passes into contacting zone 16 wherein the final hydrogenation reactions occur and is withdrawn from contacting zone 16 through line 17 whichdischarges into separator 18. In separator 18 a liquid phase and a gaseous phase separate. The gaseous phase,

which has a high hydrogen content is withdrawn through line 21. At least a portion of this gas is discharged through line 25 to control accumulation of inerts, and the remainder may be recycled through line 21 and into line 1. Line 22 entering line 21 provides for the addition of make-up hydrogen which is necessary for the process to function. The liquid phase in separator 18 passes from the lower section of the latter through line 19 and to line 1. Line 20 provides for the withdrawal of cyclohexane product from the process.

Line 24 is connected to a suitable source of benzene and passes into line 23 which is a header feeding the before mentioned lines 2, 6, 10 and 14. Suitable flow controlling devices, not shown, may be used in lines 2, 6, 10 and 14 in order to provide for the proper distri bution of benzene to the inlets of each of the contacting zones. A suitable level controlling device, not shown, may be used in separator 18 to maintain a liquid phase in the lower section thereof at all times. Other valves compressors, controllers etc., may be used in the various parts of the process where they are needed, as for example the recycle hydrogen may be circulated with a compressor and flow control valves may be used in lines 1, 9, 20, 5, 9, 1s, and 17.

Coolers 26, 27, 28, and 29 are placed in emuent lines 5, 9, 13, and 17 respectively so that the inlet temperatures of the contacting zone and the separator may be regulated. Other heating and cooling means may be used where required as for example, heaters may be used in all or a portion of the inlet lines to contacting zone 4. It is also within the contemplation of this invention to use a hydrogen purifying means in recycle line 21 to remove accumulated inerts when it is desired to recycle the total separator gas.

Any suitable hydrogenation catalyst may be used for the present invention including nickel, platinum, palladium, iron, etc., Raney nickel or any active hydrogena tion catalyst which preferably is composited with a carrier such as alumina, silica, kieselguhr, diatomaceous earth, magnesia, zirconia, or other inorganic oxides, either alone or in combination. The preferred catalyst of this invention is nickel composited with kieselguhr.

In the preferred operation, the conversion of benzene to cyclohexane will be effected at a pressure of from about 300 p. s. i. to about 500 p. s. i. It has been found that the operating pressure is of no great significance to the reaction except as to its effect on side reactions. Although a higher operating pressure would permit lower inlet temperatures by causing the reaction of hydrogen and benzene to be initiated at a lower temperature, the increased pressure would also cause destructive reactions to be initiated at a lower temperature and the net result would be that the temperature range of the entire opera tion would be lowered and narrowed. Similarly, by reducing the operating pressure, the temperature at which destructive reactions begin would be increased, however, a higher temperature would be necessary to initiate the hydrogenation reactions. The net result of reducing the operating pressure would be to raise the entire temperature range of the reaction. It may therefore be seen that the preferred pressure of this invention is one that has been found particularly suitable from the stand point of practical operation, but is in no way critical or limiting upon the broad scope of this invention. It may further be seen that by varying the operating pressure of the present process, the range of operating temperatures may be varied in accordance with the hereinbefore described limitations.

Many modifications of the hereinabove described process may be made within the broad scope of this invention, as for example: (1) With suitable modifican'ons, 2 or 3 contacting zones may be used or 5, 6, 7 or 8 or more contacting zones may be used, however, when the number of contacting zones is varied the processing conditions may be varied in order to effect the proper degree of reaction. When more contacting zones are used the amount of recycle hydrogen may be decreased and the amount of recycle cyclohexane may also be decreased. (2) The amount of hydrogen recycle may be increased or slightly diminished from the amount hereinbefore stated. There is no harm in increasing the amount of hydrogen recycle circulated through the process. Slightly diminishing the amount of hydrogen also will not harm the process, however, sufiicient hydrogen must be present at all times to have an excess of hydrogen in the contacting zones in order for the process to function properly. Hydrogen should be circulated at a rate sufficient to maintain a mol ratio of hydrogen to benzene of from about 4:1 to about 50:1. (3) Although the example used in this specification illustrates the embodiment of this invention wherein the benzene is added in 4 equal portions, it is within the scope of this invention, that for varied conditions and equipment the benzene may be added in unequal portions and may be added to all or just a portion of the contacting zones, for example, when a process is operated to use 8 contacting zones the amount of benzene added to the inlet of the final contacting zone will be less than that added to the inlet of the preceding contacting zones. The final contacting zone may have no benzene added to its inlet when it is desirable to insure complete conversion. (4) The range of temperature used in each contacting zone need not be identical with the range used in preceding or succeeding zones. When large numbers of contacting zones are employed the maximum temperature reached in the latter zones may be less than that of the preceding zones. (5) The amount of recycle cyclohexane may be increased or diminished depending upon the particular processing conditions and apparatus used. Cyclohexane may be circulated in a mol ratio to benzene charge of from about 0.5 to about 3.0 or more.

From the foregoing specification it may be seen that the process of this invention provides a means of obtaining substantially pure cyclohexane in a continuous process that it well adapted to large scale commercial operation. The amount of exothermic reaction, and hence the temperature, of each contacting zone is controlled by limiting the amount of benzene introduced into the zone thereby providing automatic control over the equilibrium established in each zone as well as preventing degradation reactions which would contaminate the product.

I claim as my invention:

1. The process of manufacturing substantially pure cyclohexane which comprises contacting a mixture of benzene, hydrogen, and cyclohexane with a hydrogenation catalyst at hydrogenafing conditions in a plurality of contacting zones, said hydrogen and cyclohexane passing in series flow from the first to the last of said contacting zones, and cooling the efliuent of each of said contacting zones except the last to a temperature of from about 325 to about 400 F. by indirect heat exchange and by admixture of benzene therewith.

2. The process of claim 1 further characterized in that the eflluent stream is withdrawn from each contacting zone at a temperature of from about 500 F. to about 600 F.

3. The process of manufacturing substantially pure cyclohexane which comprises contacting a mixture of pure benzene, cyclohexane, and hydrogen with a hydrogenation catalyst at hydrogenating conditions in a plu rality of contacting zones, said hydrogen and cyclohexane passing in series flow from the first to the last of said contacting zones, cooling the effiuent of each of said contacting zones except the last to a temperature of from about 325 to about 400 F. by indirect heat exchange and by admixture of benzene therewith, and regulating the quantity of benzene thus supplied to each of said contacting zones to control the amount of exothermic reaction effected therein so that the temperature in said contacting zones does not exceed 600 F.

4. The process of claim 3 further characterized in that equal portions of the total benzene charge are added to the inlet of each of said contacting zones.

5. The process of claim 3 further characterized in that said contacting zones are maintained at a pressure of from about 300 p. s. i. to about 500 p. s. i

6. The process of claim 3 further characterized in that said plurality of contacting zones comprises four in numher.

7. The process of claim 3 further characterized in that a portion of the liquid efiluent from the last contacting zone passes to the inlet of the first contacting zone as said cyclohexane.

8. The process of claim 3 further characterized in that a portion of the gaseous efiiuent from the last contacting zone passes to the inlet of said first contacting zone as a portion of said hydrogen.

9. The process of claim 3 further characterized in that the mol ratio of hydrogen to cyclohexane at the first contacting zone inlet is from about 4:1 to about 50:1.

10. The process of claim 3 further characterized in that the mol ratio of cyclohexane recycle to overall benzene charge is from about 0.5 to about 3.0.

11. The process of claim 3 further characterized in that said hydrogenation catalyst comprises nickel supported on kieselguhr.

References Cited in the file of this patent UNITED STATES PATENTS 2,303,075 Frey Nov. 24, 1942 2,373,501 Peterson Apr. 10, 1945 2,376,086 Reid May 15, 1945 

1. THE PROCESS OF MANUFACTURING SUBSTANTIALLY PURE CYCLOHEXANE WHICH COMPRISES CONTACTING A MIXTURE OF BENZENE, HYDROGEN, AND CYCLOHEXANE WITH A HYDROGENATION CATALYST AT HYDROGENATING CONDITIONS IN A PLURALITY OF CONTACTING ZONES, SAID HYDROGEN AND CYCLOHEXANE PASSING IN SERIES FLOW FROM THE FIRST TO THE LAST OF SAID CONTACTING ZONES, AND COOLING THE EFFLUENT OF EACH OF SAID CONTACTING ZONES EXCEPT THE LAST TO A TEMPERATURE OF FROM ABOUT 325* TO ABOUT 400* F. BY INDIRECT HEAT EXCHANGE AND BY ADMIXTURE OF BENZENE THEREWITH. 